Posted by Boston Powers on 08/25/12 - 5:15 AM
#1
HELP! I've done a lot of research on this topic, but I'm still experiencing issues with my rebuilt motor running below 1,500 RPM in gear. Here is what I've done:
Motor rebuilt with all new seals and proper gasket sealing compound and adhesives, etc.
Compression is 130 on all 3 cylinders.
Running new plugs (champion) L77JC4 & tried Champion QL78C (supposed to improve idle) Gaped .030"
New fuel lines (no leaks)
Fuel pump okay. Tried squeezing bulb when problem happens (no difference). Bulb nice & hard when float valves shut.
All coils replaced (nice 1/2" spark)
Thoroughly cleaned Carbs cleaned and rebuilt with new kits. Removed low speed jets after rebuilding just to double check and blow out again. They seem fine and are .030 factory jets.
Turned flywheel by hand in & out of gear to be sure nothing binding in lower unit - felt nice and free in gear.
I have a factory service manual and have sync'd carbs. Seems pretty simple am I missing anything? Also Cam is set as described in manual to touch 1st notch at lowest speed setting.
Timing advance moves nice & free.
Blipping the choke seems to help when I sense it dying off. Runs strong at any speed 1500 & above.
I'm about at the point of drilling out low speed jets to .032, but even if that works, seems like a work around. I'm at my wits end though and may just try that as a last resort.
Any ideas would sure be appreciated!
Posted by brooks89 on 08/25/12 - 7:05 AM
#2
Before you modify the carbs I would look the motor's ability to hold a vacuum. Your topside compression is good, but have you checked the vacuum on the lower side of the pistons? Two cycle motors pull fuel from below the piston through a port to the top side where it's compressed and ignited. When you say the motor is "rebuilt" were the piston rings replaced?
Posted by lovemaiboat on 08/25/12 - 11:18 AM
#3
i just replaced my 1976 70 horse johnson with a new e tec but before that i was at wits end with my 70 horse, my mechanic installed .035 jets in carb and the thing ran fantastic, i had low end stalling and it would also stall out when giving throttle, after carb boil out and rebuilt with .035 jets it made a great difference. good luck, btw my 1976 70 horse is for sale
Posted by Boston Powers on 08/25/12 - 12:59 PM
#4
I had all 3 cylinders bored out at a marine shop. 3 new pistons and rings, bearings etc. Everything .020 over. How does one check a motor's ability to hold a vacuum on the lower side of the pistons? Can this be done myself without special equipment. I did replace both crankshaft seals (top & bottom) by the way, but I suppose there are other places it can leak.
Thanks for the posts. Also, I guess if I end up modifying the low speed jets, I can always go back the the original size if need be.
Thank you both for the reply.